Notice how compressed the spreads between banks are, both in terms of market share and dollar value?

Facebook's valuation range is widely assumed to be $75-100 billion, with the actual issuance amount much lower, perhaps in the $20ish billion range. [UPDATE: reports now place the size at $5 billion. Although this is likely to increase as 'demand' increases]

In that range, Facebook's IPO has the ability to upend a banks' position in the equity underwriting league table for 2012. That's not good.

And then, after you've missed out on Facebook, you have to try to explain to every company who wants to raise equity, tech and otherwise, why your capital markets team isn't terrible. And that leads to the real likelihood of lost business, leading to a further slip in the league table.

Then, when Facebook inevitably comes back to issue more equity, there's even less of a chance that you'd be involved.

It may seem silly, but the league table and a banks' position on it is incredibly important to the ongoing success of their underwriting business.

And at this point, league table credit and its future effects on banks' underwriting business are really the only reason they want to be involved in the Facebook IPO.